Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - ISOREE (New insight into the origin of the Earth, its bulk composition and its early evolution)
Teaser
ISOREE is a multi-disciplinary research program combining different approaches (isotope geochemistry, experimental petrology, spectroscopy) in order to bring further constrain on the origin of the Earth and the major events that occurred very early on in its history. Four...
Summary
ISOREE is a multi-disciplinary research program combining different approaches (isotope geochemistry, experimental petrology, spectroscopy) in order to bring further constrain on the origin of the Earth and the major events that occurred very early on in its history. Four tasks have been defined in this five-year research program with the aim of :
• Better constraining the Earth mantle’s composition
• Dating the successive magmatic ocean stages on Earth
• Defining precisely the age of the Moon’s formation
• Refining the giant impact model and the Earth-Moon relationship
Understanding early mantle evolution by direct observation of the material available on Earth’s surface is difficult because very rare samples dated from the first few hundred of Myrs of the Earth’history have been found. The analysis of short- and long-lived radiogenic systems on the oldest samples are able to provide some of the best constraints on the composition of the mantle and the extent of chemical differentiation in the Early Earth. On order to build a robust model for the Earth’evolution, the composition of the staring material (the Earth’s building blocks) must be known. The meteorite classification lists many samples from different groups with distinct isotope compositions (nucleosynthetic aonmalies). These samples are believed to have been formed in different locations of the solar nebula and are also characterized by different compositions in major and trace elements and have recorded different redox conditions. These conditions play a strong role in the behavior of chemical elements (preferential affinity for the silicate or metal phases).
The Earth is the only planet harbouring life in the Solar System for about 4 billion years. This unique evolution is due to a combination of several characteristics/processes produced during its primitive evolution. The results of the ISOREE project will make it possible to better understand the origin of the Earth and help finds Earth-like exoplanets.
Work performed
Task 1. Define the Ce-Nd terrestrial correlation.
Mid-oceanic ridge basalts (MORB) and ocean island basalts (OIB) have been measured for Ce and Nd isotopes. They define a negative, linear correlation called the mantle array in a binary isotope diagram. Chondrites and modeled early-depleted reservoirs plot on the mantle array. The precise determination of the mantle array does not provide any constraint on the La/Ce and Sm/Nd ratios of the Bulk Silicate Earth (BSE).
Marine sediments formed in equilibrium with seawater are characterized by negative cerium anomaly and then develop with time radiogenic 138Ce signature. Subduction of marine sediments via plate tectonics could have modified the cerium isotope composition of the mantle. Twelve lavas from Gough Island (South Atlantic Ocean) have been analyzed. This hotspot is classified in the EM-1 group and several studied suggest the involvement of subducted pelagic sediments in the mantle source. The measured cerium isotope compositions allow us to exclude the contribution of old sedimentary material carrying a negative cerium anomaly in the mantle source.
We combined the measurement of both 146Sm-142Nd and 147Sm-143Nd in Archean samples. We reported the first high precision 142Nd analyses for granitoids and amphibolites from Swaziland and the Barberton Greenstone Belt (South Africa) (Schneider et al., 2018). We measured two orthogneisses from the poorly studied Napier complex (Antarctica). Our new results (Hf-Nd isotopes) indicate that these gneisses were formed by reworking of enriched reservoirs, which represent Hadean protocrusts.
Publications:
Bellot N., Boyet M., Doucelance R., Bonnand P., Savov I., Plank T. and Elliott T. 2018. Origin of negative cerium anomalies in Mariana arc lavas from 138Ce/142Ce-143Nd/144Nd isotopic approach. Chemical Geology, 500, 46-63.
Israel C., Boyet M., Doucelance R., Bonnand P., Frossard P., Auclair D., Bouvier A. 138La-138Ce and 147Sm-143Nd isotope composition of the main silicate reservoirs and discussion on the rare earth elements pattern of the Bulk Silicate Earth. Submitted to Earth and Planetary Science Letters.
Boyet M., Doucelance R., Israel C., Bonnand P., Auclair D., Suchorski K. Bosq C. New constraints on the origin of the EM1 component revealed by the measurement of the La-Ce systematics in Gough Island lavas. Submitted to Geochemistry, Geophysics, Geosystems.
Schneider K., Hoffmann E., Boyet M., Münker C., Kröner A. 2018. Coexistence of enriched and modern-like 142Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa. Earth Planet. Sci. Lett., 487, 54-66.
Guitreau M., Boyet M., Paquette J.-L., Gannoun A., Konc Z., Hénot J.-M., Seydoux-Guillaume A.-M. Hadean protocrust(s) reworking at the origin of the Archean Napier Complex (Antarctica). In preparation for Nature.
Task 2. Extra-terrestrial material: Age of the giant impact forming the Moon, and Ce isotope composition of meteorites and lunar samples.
We have obtained the first Nd isotope measurements on a fall enstatite chondrites (EC) from the EL3 group. We showed that EC from the EL3 and EL6 subgroups may come from different parent bodies. Samples from the EL3 subgroup have Nd and Ru isotope ratios undistinguishable from that of the Bulk Silicate Earth and their Mo isotope composition has never been measured (Boyet et al., 2018).
We have measured 11 chondrites for the La-Ce systematics. Enstatite, ordinary and carbonaceous chondrites have indistinguishable average values within error bars. We have defined the Ce isotopic composition of CHUR as being identical to the mean value of our nine, unmodified chondrite samples: 138Ce/142CeCHUR = 0.02256577 ± 66 (29 ppm; 2 s.d.). The two EL6 samples have fractionated La/Ce ratios and fall on the 4.568 Ga isochron.
A petrological and geochemical study has been realized on NWA 8486, which has been suggested to originate from Mercury. We find that minerals, in particular, pyroxenes have a uniqu
Final results
Progress beyond the state of the art and expected results until the end of the project
TASK 1. Ce-Nd isotope measurements on ocean island basalts and chondrites suggest that the bulk silicate Earth has a chondritic Sm/Nd and La/Ce composition. High-precision measurements of the 138Ce/142Ce ratio is obtained by TIMS using a combination of 1011-1012-1013 Ω amplifiers.
TASK 2. We developed a method based on a 4-mass-step acquisition scheme to measure all isotope ratios dynamically by thermal ionization mass spectrometry (TIMS). Following a comprehensive evaluation of the mass spectrometer runs, we suggest several criteria to assess the quality of data acquired by TIMS.
chondrites from the EL3 subgroup are the best candidates for the Earth’s building blocks.
The unique achondrite NWA 8486 comes from a plagioclase crust. Such anorthositic crusts have never been observed on planetesimals or within meteorite collection, beside on the Moon which is a much larger planetary body formed late in the Solar System history.
TASK 3. REE are not incorporated into the core even in reducing conditions and La/Ce is not fractionated.
Experiments performed in evacuated silica tubes show that Eu, Yb and Sm may behave differently from other lanthanides in reducing conditions. Anomalies are caused by reduction (2+ form).
U/Th ratios measured in metal/silicate experiments show that 1) EL chondrites are the best candidates for the terrestrial buikding blocks, and 2) the oxygen content of the Earth’s core must be lower than 4 wt%.
TASK 4. The development of a triple spike method for measurement for the Ce isotope composition improves the precision (2SD = 30 ppm) relative to sample/standard bracketing method.
Mass dependant stable isotope fractionations have been measured in oxidation reaction. This fraction is typical of isotopic fractionation during a kinetic reaction.The next step is to work on zircons which are formed in high temperature environment. First in syntetic zircons, then in natural samples.
Website & more info
More info: http://lmv.uca.fr/boyet-maud/.